Animal populations provide reservoirs of many diseases transmissible to man. Moreover, animals may contribute to the inorganic pollution of the ecosystem, including natural resources and nutritional environment. The interactions between man through urbanization, management of domestic animals and wildlife on the one hand, and the dynamics of animals populations, their products and wastes on the other, form a most important complex of benefits and hazards in human ecology.

In planning comprehensive ecological approaches in (a) human health and wildlife; (b) human health and animals in rural areas; and (c) human health and animals in urban areas, major categories of animals have to be considered, namely, (a) animal industries; (b) animals used for work and transportation; (c) wildlife; and (d) pet animals.

Depending on the prevailing ecological conditions, these categories of animals play a different role in primary and secondary contamination (as sources of infection, injury, poisoning and chemical pollution) of our environment, including food. Hazards and benefits must therefore be seen for the whole cycle comprising animal feed, the live animal, foodstuff, dead animal carcasses, offal and manures.